1. Field of the Invention
The present invention relates to polyorganosiloxane compositions which can be vulcanized at high temperature to silicone elastomers, i.e., can be vulcanized at material temperatures generally of between 100° and 200° C. and possibly, where necessary, ranging up to 250° C. The invention also relates to the use of these compositions for producing, in particular by extrusion, sheaths or primary insulations forming part of fire-resistant electrical wires or cables. The invention relates, finally, to the fire-resistant electrical wires or cables which are manufactured using such compositions.
2. Description of Related Art
The expression “fire-resistant electrical wires or cables” is intended to define electrical wires or cables which are required to guarantee high-quality fire performance in terms, at least, of ash cohesion and smoke density. The characteristics which must be exhibited by fire-resistant electrical wires or cables form the subject of legal regulations in many countries, and exacting standardizations have been established.
An electrical cable is composed, in accordance with the known prior art, of one or more single conductors (generally based on Cu or Al); each of these single conductors is protected by a sheath or primary insulation made of one or more concentric layers based on silicone elastomer. Around this sheath or these sheaths (in the case of a multiconductor cable) there is (are) provided one or more packing elements and/or one or more reinforcement elements based in particular on glass fibers and/or mineral fibers. Then comes the external jacketing, which may comprise one or more jackets. In the case of a multiconductor electrical cable, the packing element(s) and/or the reinforcement element(s), which is (are) arranged around the single conductors (each equipped with its primary insulation), constitute(s) a sheath which is common to all of the single conductors. Although the silicone elastomer forming part of the cables is essentially the constituent material of the primary insulation(s), it may also be present, in variable proportions, in the packing element(s) and/or in the reinforcement element(s) (constituting the common sheath in the case of a multiconductor cable) and/or in the external jacket(s).
The number of concentric layers based on silicone elastomer that constitute the sheath or primary insulation of each single conductor, and the wall thickness of each layer, will depend essentially on the requirements imposed in order to maintain operation in accordance with the provisions of the standards. Generally speaking, it is desirable to obtain such operation by using one or two layers each having, appropriately, a thickness equal to at least 0.5 mm and, preferably, to at least 0.8 mm.
In France, for example, one important standard which relates to fire resistance tests on electrical cables, and which must be met, is the standard NF C 32-070 CR1, which relates to the period of operation of cables burning under defined conditions. The fire resistance can be ascribed to the production of ash, which is required to exhibit a certain cohesion, allowing sufficient insulation to be retained for the operation of the cables. In this test, each cable sample is positioned in a metal tube, which is itself then placed in a furnace whose temperature reaches 920° C. within 50 minutes, and this temperature is subsequently maintained for 15 minutes; in the course of this test, the cable sample is subjected to regular impacts (via an impact bar which knocks against the metal tube at a rate of two strikes per minute); the test is passed if control lamps, connected to the cable, which is supplied at a nominal voltage, have not gone out at the end of the test period (that is, after 65 minutes). The standard is met if at least 80% by number of the tests carried out are passed.
Another important standard which relates to fire resistance tests, and which must also be met, is the international standard IEC 61 034, parts 1 and 2 (IEC is the abbreviation of the expression: International Electrotechnical Commission), which relates to the measurement of the density of smoke given off by electrical cables burning under defined conditions. In this test, the light transmittance is measured within a small chamber of 27 m3 obscured by the smoke produced by burning lengths of cable under the action of an alcohol flame installed under defined conditions. The standard is met if at least 60% light transmittance is obtained.
The aforementioned standards can only be met for electrical wires or cables of which at least the primary insulation materials have been the subject of particular study with regard to their inhibition of fire spread. In practice, in accordance with the known prior art, it has been observed that primary insulating materials based on silicone elastomers obtained by high-temperature vulcanization of appropriate poly-organosiloxane compositions are able to satisfy the flame spread inhibition tests. When the silicone elastomer burns, it is transformed into an ashy insulating substance which has a certain cohesion, and it emits white smoke, which originates from the self-ignition of volatile residues that are produced by the breakdown of the elastomer.
In the prior art, polyorganosiloxane compositions which can be vulcanized at high temperature to silicone elastomers have been described that comprise a polyorganosiloxane polymer which crosslinks by peroxide catalysis, fillers of fluxing type and/or of lamellar type, which may be alone or in combination with platinum and with metal oxides, so as to give rise, in the event of a fire, to the formation of an insulating ashy substance which has a certain cohesion, allowing the operating time of burning cables to be prolonged. Mention may be made of EP-A-0 467 800, which proposes the use both of ZnO (as fluxing agent) and of mica (as lamellar filler), optionally in combination with a compound of platinum and/or metal oxides such as, for example, titanium oxide and the oxide Fe3O4.
As an illustration of the prior art, in Patent Application WO 01/34696 polyorganosiloxane compositions that can be vulcanized at high temperature to silicone elastomers are set out that contain:                100 parts of an ingredient a) consisting of at least one polyorganosiloxane polymer;        5 to 80 parts of at least one reinforcing filler;        0.2 to 8 parts of an organic peroxide;        8 to 30 parts of mica;        6 to 20 parts of zinc oxide;        0 to 15 parts of at least one additive customarily used in the field of high-temperature-vulcanizing polyorganosiloxane compositions,said compositions being characterized in that they contain, in addition, as other necessary ingredients:        0.0010 to 0.02 parts of platinum, a platinum compound and/or a platinum complex;        2 to 10 parts of titanium oxide; and        50 to 120 parts of an ingredient i) consisting of at least one bulking filler.        
Other compositions are set out in Patent Application WO 01/34705 which describes polyorganosiloxane compositions that can be vulcanized at high temperature to silicone elastomers having an improved fire performance, containing:                a) at least one polyorganosiloxane polymer;        b) at least one reinforcing filler;        c) an organic peroxide;        d) mica;        e) zinc oxide;        f) optionally at least one additive customarily used in the field of hot-temperature-vulcanizing polyorganosiloxane compositions,        said compositions being characterized in that they contain, in addition, as other necessary ingredients:        g) platinum, a platinum compound and/or a platinum complex;        h) titanium oxide;        i) at least one bulking filler; and        j) at least one mineral species belonging to the wollastonite group.        
Finally, Patent Application WO 2004/064081 describes the use of polyorganosiloxane compositions that can be vulcanized at high temperature to silicone elastomers containing:    a) at least one polyorganosiloxane polymer;    b) at least one reinforcing filler;    c) an organic peroxide;    d) mica;    e) zinc oxide;    f) optionally at least one additive customarily used in the field of high-temperature-vulcanizing polyorganosiloxane compositions;    g) platinum, a platinum compound and/or a platinum complex;    h) titanium oxide;    i) at least one bulking filler; and    j) optionally at least one mineral species belonging to the wollastonite group,            said compositions being characterized in that the bulking fillers i) consist of surface-treated aluminum hydroxide Al(OH)3 powders.        
However, such polyorganosiloxane compositions that can be vulcanized at high temperature to silicone elastomers proposed to date are not completely satisfactory especially from the point of view of the preparation and use of such compositions. Specifically, these compositions have the disadvantage of exhibiting tackiness properties that thus complicate their handling (or “their processability”) during an industrial preparation or when they are used in extrusion within the context of the manufacture of electrical wires or cables. Furthermore, a compromise is still sought in terms of ceramization, low density of the smoke released by burning electrical cables, mechanical properties before and after aging and ease of use (or “processability”) of the product.